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'''Starspots''' are equivalent to [[sunspots]] but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the [[Sun]], up to about 30 % of the surface may be covered, corresponding to a size 100 times greater than the ones on the Sun. |
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* poi |
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==Detection and Measurements == |
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All 12 (2004) |
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To detect and measure the extent of starspots one uses several types of methods. |
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*For rapidly rotating stars - [[Doppler imaging]] and [[Zeeman-Doppler imaging]]. |
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:With the Zeeman-Doppler imaging technique the direction of the [[magnetic field]] of stars can be determined since light from the spots is be split according to the [[Zeeman effect]], revealing the direction of the field. The strength of the magnetic field is determined from the Zeeman splitting in [[spectral lines]]. |
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*For slowly rotating stars - Light Depth Ratio (LDR). |
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:Here one measures two different spectral lines, one sensitive to temperature and one which is not. Since starspots have a lower temperature then their surroundings the temperature-sensitive line changes in depth. From the difference between these two lines the temperature and size of the spot can be calculated, which can give temperature accuracy of 10K. |
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==Temperature== |
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Observed starspots have a temperature which is in general 500-2000 [[Kelvin]] cooler then the stellar [[photosphere]]. This temperature difference could give rise to a brightness variation up to 0.6 [[magnitudes]] between the spot and the surrounding surface. |
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There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (observed in [[G-K dwarfs]]) |
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==Lifetimes== |
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The lifetime for a starspot depends on its size. |
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*For small spots the lifetime is proportional to their size, similar to spots on the Sun. |
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*For large spots the sizes depend on the [[differential rotation]] of the star, but there are some indications that large spots which give rise to light variations can survive for many years even if the stars have differential rotation. |
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==Activity cycles== |
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The distribution of starspots across the stellar surface varies analogous to the solar case, but it differs for different types of stars, e.g., depending on whether the star is a [[binary star|binary]] or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the [[Maunder minima]] for the Sun. |
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===Flip-flop cycles=== |
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Another activity cycle is the so called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres. |
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Flip-flop phenomena are observed for both binary [[RS Canum Venaticorum variable|RS CVn]] stars and single stars although the extent of the cycles are different between binary and singular stars. |
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==References== |
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[http://cdsads.u-strasbg.fr/abs/2005LRSP....2....8B S.V.Berdyugina (2005), “Starspots: A Key to the Stellar Dynamo”, Living Reviews in Solar Physics, vol. 2, no. 8 ] |
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[http://cdsads.u-strasbg.fr/abs/1997akst.book.....S K.G.Strassmeier (1997), “Aktive sterne. Laboratorien der solaren Astrophysik”, Springer, ISBN # 3-211-83005-7] |
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Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the surface may be covered, corresponding to a size 100 times greater than the ones on the Sun.
To detect and measure the extent of starspots one uses several types of methods.
Observed starspots have a temperature which is in general 500-2000 Kelvin cooler then the stellar photosphere. This temperature difference could give rise to a brightness variation up to 0.6 magnitudes between the spot and the surrounding surface. There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (observed in G-K dwarfs)
The lifetime for a starspot depends on its size.
The distribution of starspots across the stellar surface varies analogous to the solar case, but it differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun.
Another activity cycle is the so called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres. Flip-flop phenomena are observed for both binary RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars.
S.V.Berdyugina (2005), “Starspots: A Key to the Stellar Dynamo”, Living Reviews in Solar Physics, vol. 2, no. 8 K.G.Strassmeier (1997), “Aktive sterne. Laboratorien der solaren Astrophysik”, Springer, ISBN # 3-211-83005-7
